The invention relates to an image-processing method for automated contrast modification of digital image data, in which at least a low and a high-frequency signal component are formed from the image signal of the image data; the low-frequency signal component is modified according to a characteristic-curve function; and then the modified, low-frequency signal component is then added to the high-frequency signal component.
Digital image data often include very great brightness differentials, and are therefore difficult to reproduce on photographic paper since these materials include a contrast range that is too narrow. Thus, copies of such images are over-exposed in the bright areas but under-exposed in the darker areas. Individual details are thus very difficult, or impossible, to discern in the under- or over-exposed areas. A classic example of this is a photograph of a shadowed gateway arch with an extremely bright background. Contrast modifications have been developed in order to improve the quality of such high-contrast photographs for printed pictures for such copies from digital image data.
One such procedure in which particularly the global contrast (gradation) of digital image data is modified is described in the German Patent No. DE 36 29 409 A1. This contrast modification procedure is based on subjecting the digital image data to frequency filtering in which they are split into a low-pass and a high-pass component. In order to optimize global contrast, the low-pass signals are modified based on a non-linear gradation separating line. The low-pass signals thus modified are then again added to the high-pass signals, whereby in general the high-pass signals are also modified in order, for example, to undertake focal sharpening of detail information contained in the image. Contrast modification is, however, exclusively realized within the low-pass component since the high-pass component does not allow any influence on global image sizes.
During application of such image-processing procedures for contrast modification, it has been shown, however, that undesirable effects are sometimes triggered with certain motifs. Thus, for example, a dark motif before a medium-density background may be so reproduced after image contrast modification that a very bright border arises about the dark motif that only matches the density of the background at a larger distance from the motif. Such an occurrence is called a “white halo” because of its appearance. In an opposite density constellation, so-called “black halos” may also be generated if a bright motif is to be reproduced against a medium-density background.
This problem with contrast modification procedures, in which conventional copies are made on photographic paper, has long been known. In this conventional procedure, contrast is modified by locally influencing the copy light, realized by the LED illuminating the negative, with a digitally-created mask. The mask thus reproduces a strongly-unfocused, inverted intermediary image of the motif to be copied. In this procedure, the halo phenomenon is avoided in that the very strong bright/dark interfaces that cause these phenomena are slightly displaced in the mask. This displacement results in areas in which the photographic paper is either too strongly or too weakly exposed, since the paper sensitivity to density gradations in these areas is very low. The effect is thus produced that the strong contrast transitions are displaced into almost invisible areas of densities. Such a procedure is described in the German Patent No. DE 197 03 063 C2. This approach for avoiding undesirable halos is not applicable, however, in an image-processing procedure for digital image data as described in the aforementioned DE 36 29 409 A1, since an unfocused mask which might be modified in the case of strong contrast transitions is not explicitly taken into account in this process.